Coating apparatus – Solid applicator contacting work – Movably mounted applicator
Reexamination Certificate
1999-12-15
2003-05-06
Lamb, Brenda A. (Department: 1734)
Coating apparatus
Solid applicator contacting work
Movably mounted applicator
C118S212000, C101S157000
Reexamination Certificate
active
06558466
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of roller/gravure coating. More particularly, the invention concerns a coating apparatus that meters a film of liquid coating solution from a coating surface or roller and then diverts it away, thereby preventing contamination of the coating surface.
BACKGROUND OF THE INVENTION
In conventional roller/gravure coating processes (as described, for example, in U.S. Pat. No. 4,373,443, Feb. 15, 1983, by Matalia et al., entitled, “Method Of High Viscosity Inking In Rotary Newspaper Presses” where a gravure cylinder provides ink in newspaper presses), a liquid coating composition is directed to the surface of a coating applicator roll
1
by one of several suitable means including rotating (denoted by arrow) the applicator roll
1
through a reservoir
2
of liquid
3
, as illustrated in FIG.
1
. The surface of the coating applicator roll
1
may have a smooth finish or it may be engraved with cells/grooves
5
of prescribed volume. Often, the layer of liquid
3
picked up by the applicator roll
1
from the reservoir
2
is subsequently metered to a thinner film using a doctor blade
4
. In gravure coating, for example, the doctor blade
4
removes all the applied liquid except that which is present in the engraved cells
5
formed in the gravure cylinder
1
. Alternatively, the steps of wetting (filling) and doctoring may also be combined as described in U.S. Pat. No. 4,158,333, Jun. 19, 1979, by Navi, titled, “Inking Baffle For Rotary Newspaper Presses.” After the doctoring step, the liquid remaining on the surface of a smooth coating applicator roll or that remaining in the cells
5
of an engraved coating applicator roll is transferred to a moving web
6
by impressing the moving web
6
between the applicator roll
1
and a soft backer or impression roll
7
. In
FIG. 1
, the web
6
is shown to be moving in the same direction as the surface of the coating applicator roll
1
at the point of contact between the two, but in roller/gravure coating practice, the web may be conveyed in the opposite direction as well. The thickness of coating transferred to the moving web
6
is generally a known fraction of the thickness of liquid film retained on the surface of a smooth coating applicator roll downstream of the doctoring step or, alternatively, it is a known fraction of the volume of the engraved cells
5
per unit surface area of an engraved coating applicator roll
1
.
Depicted in
FIGS. 2
a
and
2
b
, a shortcoming of existing roller/gravure coating processes is that when excess liquid
8
removed by the doctor blade
4
falls back on the surface of the coating applicator roll
1
, it is carried back up to the “bank” of coating liquid
9
that is accumulated between the moving coating applicator roll
1
surface and the stationary doctor blade
4
. Since the excess liquid
8
falls back on and contacts the surface of the coating applicator roll
1
in a turbulent and random manner, this renders the bank of coating liquid
9
uneven in the cross-web direction. The unevenness of the bank of coating liquid
9
in turn causes a coating defect in the form of streaks and bands
10
, as exemplified in FIG.
3
. The defect is especially prominent in particulate coating dispersions (as opposed to solutions).
An analysis of the nature of the flow of metered liquid
3
behind the doctor blade
4
reveals that at low coating applicator roll
1
surface speeds the liquid
3
simply runs back down the surface of the coating applicator roll
1
in a laminar fashion (see flow lines
11
in
FIG. 4
a
). However, as speed of the coating applicator roll
1
is raised, a point is reached when the metered liquid
3
separates from the surface of the coating applicator roll
1
and flows (see flow lines
12
in
FIG. 4
b
) generally along the underside
13
of doctor blade
4
and away from the surface of the applicator roll
1
.
Moreover, at some point further downstream of the contact point
14
between the doctor blade
4
and the coating applicator roll
1
, the deflected liquid loses its momentum and therefore separates from the underside surface
13
of the doctor blade
4
and falls or flows vertically downwards under the influence of gravity (refer to
FIG. 4
b
).
Presently the defect can be avoided in one of several ways. One way known to avoid this defect is to maintain the coating speed below the speed of transition from “runback” flow to “deflected” flow. Experimental observations indicate that the speed of transition between runback flow (
FIG. 4
a
) and deflected flow (
FIG. 4
b
) depends on operating parameters—viscosity and surface tension of liquid; tangent angle between doctor blade
4
and surface of the coating applicator roll
1
; thickness of the incoming film of liquid; radius of coating applicator roll
1
; etc. Here, runback flow is defined as the case where liquid removed by the doctor blade
4
runs back down the surface of the coating applicator roll
1
. Deflected flow is where the excess liquid
8
metered by the doctor blade
4
travels away from the surface of the coating applicator roll
1
, along the underside
13
of the doctor blade
4
, up to a point where it loses its momentum, and then further separates from the underside
13
of the doctor blade
4
surface, and drops vertically under the influence of gravity.
Unfortunately, under normal operating/manufacturing conditions, the speed of transition from runback to deflected flow is too low for it to be a practicable production speed.
Referring to
FIGS. 5
a
and
5
b
, another known way to avoid the defect is to locate the contact point or tip
14
of the doctor blade
4
at application points on the cylindrical coating applicator roll
1
surface that are far from top-dead-center
19
. Then, especially in the case of small diameter cylinders, i.e., typically diameters less than about 5 inches, the deflected excess liquid
8
in all likelihood will not flow back to the cylindrical coating applicator roll
1
surface on its way down (refer to
FIG. 5
b
). But at application points close to top-dead-center
19
, and with large diameter coating applicator rolls
1
, the excess liquid
8
will tend to flow back to the surface of the coating applicator roll (
FIG. 5
a
).
Unfortunately, the location of the contact point or tip
14
of the doctor blade
4
, relative to top-dead-center
19
cannot be changed arbitrarily. For instance, to minimize evaporation of coating liquid
3
from the surface of the coating applicator roll
1
in the region between the contact point or tip
14
of the doctor blade
4
and top-dead-center
19
, it may be necessary to narrowly fix the distance of the contact point or tip
14
of the doctor blade
4
from top-dead-center
19
. Similarly, the diameter of the coating applicator roll
1
may also have to be narrowly fixed. This is true, for instance, in the coating of discrete patches or patterns using gravure coating, wherein the ratio of gravure cylinder circumference to engraved patch/pattern length has to be maintained constant.
While there are no known prior art attempts to solve Applicants' specific problem of diverting coating liquid from the surface of a coating applicator roll having an excess quantity of liquid thereon, U.S. Pat. No. 5,755,883, May 26, 1998, by Kinose et al., titled, “Roll Coating Device For Forming A Thin Film Of Uniform Thickness” discloses a roll coater having a blade scraper for scraping coating liquid from a metal roll and a tray positioned beneath the nip for catching the scraped liquid. This device provides only for preventing fluid from contacting coating elements beneath the nip and does not protect the roll from which the liquid was deposited from receiving excess liquid.
An attempt to use a similar tray in a location between the underside
13
of the doctor blade
4
and the surface of the coating applicator roll
1
(refer to
FIG. 6
) was not successful because there is very little room available there. Indeed the deflected excess liquid
8
separates from the underside
1
Fitzgerald Barry A.
Hanumanthu Ramasubramaniam
Lobo Rukmini B.
Bailey, Sr. Clyde E.
Eastman Kodak Company
Lamb Brenda A.
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